Catalytic reforming processes have long served as a primary means of upgrading hydrocarbon streams to increase the octane of gasoline products.
Generally the hydrocarbon feedstock comprises a petroleum gasoline fraction commonly referred to as naphtha. The naphtha stream usually comprises relatively large concentrations of naphthenic and substantially straight, chain paraffinic hydrocarbons. The reforming process performs a variety of concomitant reactions which consists principally of naphthene isomerization, dehydrogenation of naphthenes to aromatics, dealkylation and demethylation of aromatics to lighter aromatics, isomerization of normal paraffins to isoparaffins, and hydrocracking. Reforming is a catalytic process that relies on a substantial number of acid and metal sites on the catalyst. A typical reforming process mixes hydrogen with the hydrocarbon feedstock before entering a first reaction zone. The feed passes serially through at least one additional reaction zone before separation to provide a vapor phase comprising hydrogen for recycle of the feedstock and a liquid product phase providing the gasoline composition. Since the various reactions that take place are highly endothermic, the process takes place in a series of reaction zones with intermediate reheating between the reaction zones to maintain reaction temperatures. It has been taught that the reforming process can operate at a wide variety of conditions including temperatures in a range of from 800.degree.-1100.degree. F., pressures of from 0 to 1000 psig, liquid hourly space velocities (LHSV) of from 0.1 to 10, and hydrogen to hydrocarbon ratios of from 0.5 to 20.
Most reforming processes are designed to operate in either a continuous manner with a continuous catalyst regeneration section, or in what is generally referred to as a semi-regenerative operation. Reforming operations started out with a semi-regenerative process in which feed to a reactor or reactors is periodically stopped and the catalyst within that reaction zone is regenerated by steps of coke oxidation, halogenation, and reduction. Continuous catalyst regeneration is well known in the art and is used to continuously or intermittently remove small amounts of catalyst from an operating series of reforming reactors without stopping the feed to the series of reaction zones and returning the catalyst to the series of reaction zones after regeneration. Continuous catalyst regeneration operations have generally been favored in the past to permit the operation of the reformer at high severity and thereby maximize octane production. Where a reforming process operates at low severity, there is a disincentive to the use of a continuous catalyst regeneration system due to its added cost.
Historically, reformers have operated on naphtha-rich feeds to produce aromatics and in particular benzene with the additional benefit of producing large amounts of hydrogen through dehydrogenation reactions. In the past, a typical gasoline fraction for upgrading in a reforming process had initial boiling points of from 150.degree.-200.degree. F. and end boiling points of from 325.degree.-425.degree. F. New requirements for reformulated gasoline have profound impacts on the operation of reformers. Reformulated gasoline requirements impose limitations on gasoline end points, benzene as well as total aromatics, and reid vapor pressure (RVP). Benzene reduction poses one of the most severe problems for the operation of the reformer for benzene precursors must be removed prior to a reforming operation or removed from the product stream by either direct saturation or saturation through isomerization. In some cases benzene can be reduced by alkylation to cumene. The most advantageous way to eliminate benzene is by minimizing its production within the reformer. Benzene and benzene precursors may be removed prior to reforming or benzene may be treated after the reforming operation by saturation directly or through isomerization, alkylation to cumene or other higher aromatics or finally by direct extraction to recover benzene products where refiners are equipped to provide petrochemical benzene. Limitations on the amount of aromatics arise indirectly due to limitations on benzene and the function of aromatics as benzene precursors. Therefore, along with benzene reduction, reduction in aromatics will also follow from the revised role of reforming under the requirements of reformulated gasoline. Other effects of reformulated gasoline requirements such as limitations on end points and RVP will have less direct impact on the operation of the reforming zone but will nevertheless impact the operation severity of reforming units. However, requirements for oxygen containing compounds in the gasoline pool will increase gasoline octane via the addition of alcohols and ethers such as MTBE. These relatively high octane blending components reduce the overall octane requirement of the reformer thereby further altering operations. Thus, reformulated gasoline will have the effect of lowering octane requirements along with hydrogen production from the reformer.
Unfortunately, the lower production of hydrogen comes together with increased demands for hydrogen in most refineries. For example, reformulated gasoline will also emphasize the need to remove sulfur from feedstocks often through hydrodesulfurization. Increased hydrogen demands further arise from the hydrocracking of heavy naphtha components which come available from the decrease in the ASTM 90% distillation point (T-90) and the additional needs of sulfur removal. The necessity for direct saturation of benzene will require additional amounts of hydrogen. There will also be increased emphasis on isomerization and alkylation to provide lower RVP components to the gasoline pool which, with the concomitant side reactions, also consume more hydrogen.
External sources of hydrogen are available such as hydrogen plants, or other technologies for supplying hydrogen, however these methods are not favored due to their relatively high cost. Thus, there is a need for processing reforming feeds that will accommodate the requirements for reformulated gasoline. Such a method needs to provide high hydrogen production at low octane severity with a low production of benzene. An additional requirement is that such process operate without an increase in RVP.